The parent application Ser. No. 391,271 is based on the idea to enable a power transmission from the power input member to the power output member in a purely mechanical manner without a servo-support, through the expedient of a lost-motion coupling between the power input member and the control valves, at low force expenditure. By means of an elastic, resilient coupling member mounted in the power-flow between the power input member and the control valves, it should furthermore be possible to gradually build up a mechanical-hydraulic power transmission in addition to the mechanical power transmission in the sense of an increasing reaction power and therewith also in increasing the servo-support.
Two embodiments were disclosed in the parent application for such an arrangement. In one embodiment, two springs operate in opposite directions between an actuating member and an individual control piston. In the second embodiment, two springs function between one lever which serves as actuating member and a second lever which affects two control pistons which are mounted transverse and off-center to its axis. Both cases have the similarity in that a longitudinal change of spring (or deformation of its extension) results necessarily in an opposite change of the other spring. In cases where the springs were hard or stiff enough, it was possible, to a certain extent, to increase the characteristic curve of the input power over the output power, however, there are recognized certain disadvantages in the utilization of such hard springs in the area of servo-control systems. These disadvantages consisted in that in steering-operations with purely mechanical power transmissions, namely, in pre-stressed coupling means, as well as during an outer reverse position of the wheels, it is necessary to build up a substantial fluid pressure in the servomotor in order to move the control piston, or one of a plurality of control pistons, via the inherent reaction area in a position which would release the fluid outflow. A further situation existed in the fact that it is difficult -- on the basis of the mutually-releasing spring forces in a neutral position -- to produce a predetermined initial fluid pressure in control valves having a four-way control and positive overlapping. Such an initial fluid pressure at a valve, for example, is however desirable in order to avoid negative pressure during steering-processes, to reduce noise, and to enable a dampened operation of the servopistons.